Power supplies for lighting systems typically comprise a rectifier inverter system for converting an incoming mains voltage to a high frequency.
FIG. 1 shows a low voltage illumination system designated generally as 10 as described in U.S. Pat. No. 6,097,158 (Manor et al.) commonly assigned to the present assignee and incorporated herein by reference. The illumination system 10 comprises a pair of input terminals 11 and 12 for connecting to a source of low frequency AC voltage 13 shown in dotted outline. The AC voltage source 13 is derived from a conventional electricity supply feeder having a typical mains voltage of 347-100 V and a supply frequency of 50/60 Hz. A conventional rectifier 14 is coupled via the terminals 11 and 12 to the source of AC voltage 13 for converting the low frequency AC voltage to DC which is then fed to an inverter 15 containing a conventional chopper circuit for converting to high frequency AC at 30 KHz. The rectifier 14 in combination with the inverter 15 thus constitutes a frequency conversion means 16 for converting the low frequency AC voltage to high frequency AC voltage.
A step down transformer 17 is coupled to an output of the frequency conversion means 16 for converting the high frequency supply voltage of 347-100 V to high frequency, low voltage AC signal having low voltage 48 V or below, typically 12 V. The step down transformer 17 is preferably implemented using a toroidal ferrite core and the output winding is preferably implemented using a litz (bundle of very fine insulated wires) in order to minimize losses by reducing the leakage current due to the air gap between the primary and secondary windings and by reducing losses due to the skin-effect and proximity effect. Other cores and windings can also be used. Alternatively a higher frequency may be generated and the output transformer implemented using a planar transformer.
In this prior art, albeit not in conventional prior art, to prevent the drawback associated with large high frequency currents, the high frequency signal is rectified using a synchronous rectifier 18 coupled to a secondary winding (not shown) of the step down transformer 17 for converting the low voltage AC to low voltage DC. A pair of conductors 19 and 20 are connected to the low voltage DC for connecting low voltage lamps (not shown) thereto.
FIG. 2 shows a known ignition circuit 30 for an AC-DC or AC-AC inverter 31 that is coupled to the output of a bridge rectifier 32 and whose ignition is based on an RC circuit 33 and a trigger diode 34, used for instance for powering a low-voltage filament lamp 35. The RC circuit 33 includes a capacitor 36 that is charged via a resistor 37. Upon the trigger diode reaching a breakdown voltage, the capacitor 36 is discharged through a drive transformer (not shown), leading to ignition.
Also shown in FIG. 2 is a dimmer 38 whose output is coupled to the input of the bridge rectifier 32 for varying the brightness of the lamp 35. When the inverter 31 is used with a leading or forward edge control switch (F-dimmer), in parallel to the RC circuit 33, an accelerator circuit 39 is coupled to the output of the bridge rectifier and feeds an acceleration signal to the inverter 31 to speed up the ignition process, thus leading to a better synchronization of the ignition process with the dimmer's cut-on.
It is important to note that in such schemes the inverter is not active between the dimmer cut-off and following cut-on. This leads to the absence of a load on the dimmer, which is a drawback of this dimmer-inverter system. Additional drawbacks relate to the instability of the switching moment relative to the zero crossing of the input voltage, which depends on the inverter load, length of connecting wires, capacitance of the input filter, capacitance in the inverter's input bridge, etc.
Moreover, as is explained below in greater detail, the presence of the passive state of the inverter prior to ignition causes a number of parasitic processes which desynchronize the inverter and destroy the normal functioning of the dimmer, which in turn harm the functioning of the whole dimmer-inverter system.
It is also known that the presence of sharp current fronts in operation of the dimmer is one of the causes of mechanical vibration of the lamp, which leads to acoustic noise. Various methods are known to reduce noise based on shaping of the forward front of the leading edge dimmer, or on utilizing the energy stored in a large capacitor for spreading the backward front in the case of the trailing dimmer. In the latter case, during the cut-off of the backward front there arises an additional current in the capacitor during the time of its discharge which leads to large mechanical vibration of the capacitor which again causes acoustic noise. As a result, reduction of the acoustic noise in the lamp is replaced by acoustic noise in the capacitor.
An additional drawback of the dimmer inverter system is the fact that the inverter must be designed to work either with the leading edge dimmer or the trailing dimmer, or must be provided with a circuit that is able to determine the dimmer type and can change its operation accordingly. However, if the dimmer type is determined incorrectly, very high acoustic noise and large shocks can arise in inverter circuits. For instance, it may happen that the leading edge dimmer will function without the shaping of the forward front with a large capacitance in the input bridge, which will lead to additional currents in the inverter and dimmer and large vibration and acoustic noise of the capacitor.
WO 03/058801 published Jul. 17, 2003 in the name of the present applicant and entitled “Lamp transformer for use with an electronic dimmer and method for use thereof for reducing acoustic noise” discloses a controller for reducing acoustic noise produced during use of a leading edge dimmer. A leading edge controller responsive to an input voltage fed thereto produces a control signal upon detection of a leading edge and a linear switch is coupled to the leading edge controller and is responsive to the control signal for linearly switching the input voltage so that a rate of rise of the leading edge is decreased. A trailing-edge controller may be coupled to a leading-trailing edge detector so as to be responsive to detection of a trailing edge dimmer for disabling the leading edge controller and decreasing a rate of decline of the trailing edge of the input voltage by using, for example, a large capacitor, as described earlier.
FIG. 3 shows schematically a further dimming problem that is associated with the connection of the inverter 31 to the output of the bridge rectifier 32 in the circuit shown in FIG. 1. The input to the inverter is capacitive owing to the presence of a large smoothing capacitor 40 that is typically connected across the output of the bridge rectifier. The input to the bridge rectifier is also capacitive owing to the presence of an EMI filter 41 across the supply output. During the inactive part of the period, i.e. when the inverter is not conductive, the capacitor 40 is charged and causes ignition to be late and unstable. In addition, charge on the capacitor 40 may trigger ignition of the inverter prior to ignition of the dimmer. This may cause several undesired scenarios:                The inverter may cause early ignition of the dimmer and change its ignition angle;        By the time the dimmer ignites, the inverter switches off, not having enough energy to sustain normal operation. Owing to the required latency, it will re-ignite late;        The early ignition of the inverter, having a nature of a fluctuation, may cause a spike in the output of the dimmer which may in turn lead to another unwanted re-ignition of the inverter.        
All these processes, being dependent on a multitude of external parameters such as ignition angle, inverter load, ambient conditions, etc. will lead to unstable operation of the system, when a dimmer is connected, in one of the described modes.
Furthermore, when the inverter is used with a leading edge dimmer, an accelerator circuit is employed to speed up the ignition process. In such schemes the inverter is not active between cut-off and subsequent cut-on of the dimmer. This leads to a loss of load on the dimmer, which is undesirable since it created flickering at the lamp and it enhances dimmer noise.
It is commonly known that shock currents are created in AC-AC and AC-DC converters during start-up, when such converters are used to power filament lamps, or any other lamp with starting characteristics similar to filament lamps. These currents are caused by the fact that the resistance of cold lamps is very low so that the converter works with what is effectively a short-circuited load. These shock currents reduce expected life of the lamp. Peak currents can reach high values.
FIG. 4 shows graphically a waveform of a soft start voltage VCS derived from a soft capacitor CS that is applied to a switching MOSFET and an output voltage Vmo of an arithmetic circuit that calculates an output voltage that is a function of the output voltage of a boost converter that forms part of the power factor correction circuit. The output voltage Vmo follows the AC line voltage and represents an envelope that is sampled using pulse width modulation (PWM) when the voltage VCs across the soft capacitor intersects the envelope. FIG. 4b shows graphically a waveform of successive current spikes that are fed by the soft start circuit to the inverter and the average input current. Thus, it is seen that the instantaneous inverter voltage follows the line voltage, but since only discrete samples of the line voltage are fed to the inverter at time intervals dependent on the duty cycle of the PWM, the average inverter voltage is lower than the line voltage. Two properties emerge from this: first, during any given AC half cycle, repeated voltage pulses are fed to the inverter; and secondly the amplitude of each voltage pulse is equal to the instantaneous peak voltage of the line voltage at the time that the line voltage is sampled.
From the foregoing it emerges that control of prior art lamp power supplies requires customized control of the inverter, thus militating against use of off-the-shelf prior art inverters. Likewise, the problems associated with shock currents caused by ignition of filament lamps allow for improvement in the soft start circuit used to reduce these phenomena. Furthermore, so far as power supplies that operate with dimmers are concerned, there remains the problem of acoustic noise whose reduction is amenable to further improvement; and the discontinuous ignition of the inverter and resulting instability of the inverter-dimmer-load system calls for improvement.